In cultivation of tomato plants, insect damage by root-knot nematodes is a serious problem worldwide. Plant bodies infected with the root-knot nematodes grow poorly or may wither, for example. Thus, root-knot nematode damage leads to the reduction in the total yield of tomato plants, and the amount of damage is estimated to reach at least 100 billion yen (Non-Patent Document 1). Accordingly, with the aim of preventing the infection with root-knot nematodes, heat treatment of soil, sterilization of soil using a root knot nematocide, etc. are performed to exterminate the root-knot nematodes. These methods, however, require a large amount of labor and cost.
In recent years, in order to address such problems, attempts have been made to breed tomato cultivars resistant to various root-knot nematodes utilizing root knot nematode resistance genes. Specifically, it has been reported that tomato cultivars resistance to the southern root-knot nematode (Meloidogyne incognita), the peanut root-knot nematode (Meloidogyne arenaria), the javanese root-knot nematode (Meloidogyne javanica), and the like were bred utilizing a resistance gene Mi-1 derived from Solanum peruvianum. However, it has been revealed that the tomato cultivars having the resistance gene Mi-1 have a problem in that the resistance gene Mi-1 does not function in an environment at a soil temperature of 28° C. or higher (Non-Patent Document 2).
On this account, further research has been made on resistance genes that function without depending on temperature conditions. Root-knot nematode resistant wild tomato species S. peruvianum PI270435, PI1126443, LA1708, and LA2157 have resistance genes that do not depend on temperature conditions, and Mi-2, Mi-3, Mi-4, Mi-5, Mi-6, Mi-7, Mi-8, Mi-9, etc. have been reported as resistance genes derived therefrom. Also, it has been reported that, among these resistance genes, Mi-3 is located on chromosome 12 and Mi-9 is located on chromosome 6 (Non-Patent Documents 1, 3, 4, and 5). However, S. peruvianum plants having these resistance genes show low cross-compatibility with Solanum lycoperiscum plants cultivated generally as edible tomatoes. This makes the seed production of S. lycopersicum plants having the above resistance genes very difficult.
Under these circumstances, only the resistance gene Mi-1 obtained by selecting root-knot nematode resistant individuals after crossing and embryo culture is used in commercially available varieties of tomato plants at present, although the root-knot nematode resistance conferred by the resistance gene Mi-1 has a problem of the above-described temperature dependence.